The present invention relates to a catalyst for a fuel cell containing a polymer solid electrolyte; and to a method for producing the catalyst. More particularly, the present invention relates to such a catalyst exhibiting excellent resistance to catalyst poisoning attributed to carbon monoxide and containing platinum and ruthenium in combination on a carrier therefor; and to a method for producing the catalyst.
A fuel cell containing a polymer solid electrolyte has become of interest as a power source of an electric automobile or a spacecraft, because such a cell is compact and provides higher current density as compared with a phosphoric acid fuel cell.
The fuel cell containing a polymer solid electrolyte has a layered structure in which a polymer solid electrolyte is sandwiched between a hydrogen electrode (anode) and an air electrode (cathode). Each the hydrogen electrode and the air electrode comprises a mixture of a precious metal-on-carrier catalyst and a solid electrolyte. In this structure, hydrogen gas supplied to the hydrogen electrode passes through micropores in the electrode, during which time the hydrogen gas contacts the catalyst and releases electrons by the action of the catalyst, to thereby be transformed into hydrogen ions. The hydrogen ions pass through the electrolyte in the hydrogen electrode and a solid electrolyte between the hydrogen and air electrodes, and reach the air electrode. At the air electrode, the hydrogen ions produce water as they react with oxygen supplied to the air electrode and electrons which flow into the air electrode from an outside circuit. Meanwhile, the electrons released from hydrogen pass through the catalyst carrier in the hydrogen electrode to the outside circuit and then flow into the air electrode. As a result, in the outside circuit, electrons flow from the hydrogen electrode to the air electrode, to thereby enable utilization of electric power.
As the hydrogen gas supplied to the hydrogen electrode, a hydrogen gas obtained through conversion from a liquid fuel such as methanol is considered promising, from the viewpoints of easy handling and high energy density. However, the hydrogen gas obtained through conversion contains a trace amount of carbon monoxide, which acts as a catalyst poison. Deactivation of a catalyst by poisoning adversely affects the characteristics of the fuel cell.
The catalyst which contains platinum and ruthenium in combination on a carrier has conventionally been known to exhibit excellent resistance to catalyst poisoning attributed to carbon monoxide. A possible explanation for the platinum-ruthenium catalyst""s resistance to catalyst poisoning attributed to carbon monoxide is that the poisonous carbon monoxide is eliminated by the following mechanism: OH ions are bonded to ruthenium because ruthenium is a hydrophilic substance, and the OH ions on ruthenium oxidize carbon monoxide adsorbed onto platinum. Accordingly, in platinum-ruthenium catalysts, in order to maximize the effect of ruthenium; i.e., resistance to catalyst poisoning attributed to carbon monoxide, platinum particles and ruthenium particles on the carrier are preferably brought as close to one another as possible.
Conventionally, catalysts used for a fuel cell containing a polymer solid electrolyte in which metallic platinum particles and metallic ruthenium particles are carried in combination by a carrier have been prepared by the following method: an aqueous solution of a platinum compound and an aqueous solution of a ruthenium compound are mixed, and carbon powder serving as a carrier and a reducing agent such as ethyl alcohol are added thereto, to thereby reduce platinum ions and ruthenium ions so as to precipitate platinum particles and ruthenium particles on the carbon powder. The proportions of platinum and ruthenium to be carried on a carrier has typically been 1:1.
However, precious metal particles are very small, having diameters on the order of angstroms. Therefore, it is quite difficult to obtain, over the entire surface of the carrier, a regular, orderly arrangement of the particles such that platinum particles and ruthenium particles are in proximity to one another. Especially, when the proportions of platinum and ruthenium on a carrier are 1:1, platinum particles and ruthenium particles are not necessarily in proximity to one another, and upon occurrence of even the slightest segregation of the particles there may be produced a portion where platinum particles and ruthenium particles are present apart from one another on the carrier. In such a case, in the region where ruthenium particles are sparsely present, the catalyst cannot exhibit sufficient resistance to catalyst poisoning attributed to carbon monoxide, resulting in catalyst deactivation with failure to exhibit satisfactory performance as an electrode catalyst for a fuel cell containing a polymer solid electrolyte.
An object of the present invention is to provide a platinum/ruthenium catalyst for a fuel cell containing a polymer solid electrolyte, wherein platinum particles and ruthenium particles are carried on a carrier such that the particles of the two species exist in proximity to one another without forming an aggregate, which catalyst exhibits excellent resistance to catalyst poisoning attributed to carbon monoxide. Another object of the present invention is to provide a method for producing the catalyst.
In an attempt to attain these objects, the present inventors have examined conventional catalysts for a fuel cell containing a polymer solid electrolyte, as well as methods of producing the same, from the following two viewpoints; proportions of platinum and ruthenium carried on a carrier, and a manner of carrying platinum and ruthenium, and in connection with each viewpoint have encountered a new finding.
The catalyst for a fuel cell containing a polymer solid electrolyte, which is a first invention disclosed by the present inventors, comprises platinum and ruthenium, and a carrier therefor, wherein the proportions of platinum and ruthenium, as measured in the completed catalyst product, are 2-4:8-6 on a mol basis.
In the present invention, the probability at which ruthenium particles exist in proximity to platinum particles is increased by means of incorporating ruthenium at a proportion higher than in the conventional catalysts, and as a result, ruthenium and platinum can be held on a carrier such that ruthenium particles are found in the vicinity of every platinum particle.
Within the above-described range, the proportions of platinum and ruthenium are most preferably 4:6, in order to easily achieve the conditions in which platinum and ruthenium are in proximity to one another on the carrier and to avoid excessive consumption of platinum, which is an active species in an electrode reaction of a fuel gas. A platinum/ruthenium catalyst produced at this ratio exhibits improved resistance to catalyst poisoning attributed to carbon monoxide and has the same catalytic activity as that of a conventional catalyst containing platinum and ruthenium in equal proportions on a carrier.
In view of application of the catalyst to a fuel cell containing a polymer solid electrolyte, the carrier bearing platinum and ruthenium at the above-described proportions is preferably a carbon powder which satisfies the definitions provided in claim 2; that is, the carrier is preferably carbon powder having micropores of a diameter of 60 xc3x85 or less in an amount of 20% or less with respect to the entirety of micropores and a specific surface area of 600-1200 m2/g.
The micropore distribution is such that micropores of a diameter of 60 xc3x85 or less are limited to 20% or less of the entirety of micropores, because a solid electrolyte cannot enter micropores of a diameter of 60 xc3x85 or less. Therefore, even if platinum particles are held in such micropores, hydrogen ions released through electrode reaction are not transferred to a solid electrolyte in the electrode and hydrogen ions cannot reach an air electrode. In other words, such a limitation on micropore distribution of the carrier assures efficiency in use of the catalyst.
The specific surface area is limited within the range of 600-1200 m2/g, because when a carbon powder having a specific surface area of 600 m2/g or more is used, a highly dispersed state of precious metal particles can be obtained, whereas when a carbon powder having a specific surface area of 1200 m2/g or more is used, micropores of a diameter of 60 xc3x85 or less account for 20% or more of the total micropores. Namely, the limitation of the specific surface area in the above-described range can provide a certain efficiency in use of the catalyst and improves the catalytic activity per unit weight of the catalyst.
In the present invention, the bimetallic catalyst for a fuel cell containing a polymer solid electrolyte, wherein platinum and ruthenium are contained in combination on a carrier, exhibits more excellent resistance to catalyst poisoning attributed to carbon monoxide under the condition that the two metals are alloyed, as described in claim 3. The catalyst containing alloyed platinum and ruthenium can be produced by application of heat treatment to the catalyst. Alloying by heat treatment can be carried out within a temperature range of 600-900xc2x0 C.
Next, the present inventors studied a method for producing a catalyst for a fuel cell containing a polymer solid electrolyte, which catalyst contains platinum particles and ruthenium particles in proximity to one another on a carrier, irrespective of the proportions thereof, and as a result obtained a new finding. The method disclosed by the present inventors for producing a catalyst for a fuel cell containing a polymer solid electrolyte includes the following steps. Platinum is held on a carbon powder which serves as a carrier to thereby produce a catalyst, and the catalyst and an aqueous solution of a ruthenium compound are mixed. To the resultant mixture, an alcohol is added in an amount of 5-15 vol. % with respect to the total volume of the mixture, and the resultant solution is heated, to thereby reduce ruthenium. As a result, platinum and ruthenium are held on the carrier in combination.
The present invention was achieved on the basis of the following findings of the present inventors, who carried out careful studies on conventional methods for producing a catalyst for a fuel cell containing a polymer solid electrolyte.
When ruthenium ions in an aqueous solution are reduced by a reducing agent, if the product of the concentration of the reducing agent and the concentration of ruthenium ions is insufficient, oxidation reaction induced by the reducing agent does not occur, and therefore, ruthenium particles do not precipitate. However, even when the concentration of the reducing agent is low, if a platinum catalyst co-exists in the solution, the reducing agent is oxidized by the oxidizing action of platinum, in the vicinity of platinum particles, and electrons are released. The released electrons act on the ruthenium ions, to thereby reduce the ruthenium ions to metallic ruthenium, precipitating the metallic ruthenium on the carrier. In this event, electron donating reaction occurs only in the vicinity of the platinum particles where oxidation power prevails, and therefore, metallic ruthenium precipitates in proximity to the platinum particles.
A basic scheme of the present invention may be summarized as follows. First, a platinum catalyst is produced. The platinum catalyst is mixed with a solution of ruthenium. Thereafter, under a suitable concentration of a reducing agent, electron donating reaction between the reducing agent and ruthenium mediated by the platinum is utilized with priority. By this method, there can be obtained a composite catalyst in which the two precious metals are carried in a highly dispersed state as compared with the case of conventional methods.
The method of the present invention for producing a catalyst for a fuel cell containing a polymer solid electrolyte will next be described in detail. First, platinum micro-particles are caused to be supported by a carrier to thereby obtain a platinum catalyst. An example method for producing the platinum catalyst is adding carbon powder serving as a carrier to an aqueous solution of a platinum compound, mixing, then adding a reducing agent, and mixing again, to thereby reduce platinum particles. Examples of the aqueous solution of a platinum compound include an aqueous solution of dinitrodiamineplatinum nitrate and an aqueous solution of chloroplatinic acid. Examples of the reducing agent include sodium borohydride, alcohol, and hydrogen gas, and among these, alcohol is preferred, with ethyl alcohol being particularly preferred.
Next, the platinum catalyst is added into an aqueous solution of a ruthenium compound. Examples of the aqueous solution of a ruthenium compound include aqueous solutions of ruthenium chloride, ruthenium nitrate, and a ruthenium complex, and among these, an aqueous solution of ruthenium chloride (RuCl3) is preferred.
To the resultant mixture, alcohol is added as a reducing agent for ruthenium ions. As described above, the most notable characteristic feature of the present invention resides in the point that the reducing agent is oxidized by the oxidation power of the platinum catalyst and ruthenium ions are reduced by discharged electrons. In the present invention, this reaction is induced by suitable selection of alcohol species, alcohol concentration, reaction temperature, and reaction time.
Alcohol concentration is preferably 5-15 vol. % with respect to the total volume of the mixture. When the concentration is below the lower limit, ruthenium particles are difficult to reduce by the oxidation power of the platinum catalyst, whereas when concentration is higher than the upper limit, spontaneous reduction due to reducing power of the alcohol itself occurs even at a location other than the vicinity of platinum particles, leading to aggregation of ruthenium particles.
Examples of alcohol species which may be used include methyl alcohol, ethyl alcohol, propyl alcohol, and butyl alcohol. Of these, methyl alcohol and ethyl alcohol are most preferred, from the viewpoint of reducing power. Regarding the reaction temperature, the present invention may be carried out at comparatively low temperature; about 50xc2x0 C. However, from the viewpoint of shortening reaction time, the reaction temperature is preferably near the boiling point of the mixture to which alcohol has been added. Generally, the reaction temperature is 90-100xc2x0 C. Reaction time is preferably two hours or longer. The reason for this is that when the reaction time is shorter than two hours, reduction of ruthenium particles is incomplete, which may cause aggregation of ruthenium particles in the subsequent heat treatment for alloying. In this connection, even when the reaction time is longer than two hours, there cannot be obtained a commensurable improvement in the dispersion state of ruthenium.
The bimetallic catalyst containing platinum and ruthenium in combination on a carrier has another embodiment in which the catalyst is subjected to heat treatment for alloying so as to bring metal particles of the two species in closer proximity to one another. When alloying is performed, the catalyst exhibits even further improved resistance to catalyst poisoning attributed to carbon monoxide. Alloying by means of heat treatment is preferably carried out at 600-900xc2x0 C. Temperatures below 600xc2x0 C. result in incomplete alloying of precious metal particles, whereas temperatures higher than 900xc2x0 C. promote aggregation of catalyst particles, providing an impermissibly large particle size and adversely affecting the catalyst activity.
In the above-described method for producing a catalyst for a fuel cell containing a polymer solid electrolyte, no particular limitations are imposed on the carrier species serving as a starting material or on the proportions of platinum and ruthenium on a carrier. However, when the proportions of platinum and ruthenium are set to 2-4:8-6, the catalyst exhibits further improved resistance to catalyst poisoning attributed to carbon monoxide.